1. Field of the Invention
This invention is addressed to measurement and control of the water or filler content of sheets and bands of dielectric materials such as sheet materials including paper, plastics, textiles and wood veneers, the water content measurement of bands of filamentary material such as plastic and textile fibers and the water content measurement of food and agricultural products during pre-storage processing. Measurement and control of water content is of primary importance in the manufacture of sheet materials such as paper and plastics. The same basic technique can also be used for fillers which are added to improve strength or other qualities. Although the following discussion is framed mostly in terms of water content, it is also applicable to the measurement of such fillers.
2. Discussion of Background
In order to establish the utility of a particular water content measuring apparatus, it is necessary for the particular apparatus to display a wide range of water content which it can accommodate and to provide a further wide range with respect to the kind of dry stock which it can accommodate. Other important characteristics of the apparatus which is used for measuring water content include the sensitivity to non-moisture related stock characteristics, the spatial resolution and the ease and speed with which the material can be scanned.
Prior art attempts at measuring and controlling the humidity and moisture are contained in, for example, "Humidity and Moisture: Measurement and Control in Science and Industry", Volume II, Section II, Editors, A. Wexler & E. J. Amdur, Symposium Proceedings, Reinhold Publishing Corporation, New York Library of Congress #65-13613 (1965) and in the "Paper Industry Instrumentation", J. R. Lavigne Miller Freeman Publishers, San Francisco, 1977, Chapter 10. These techniques usually employ either infrared radiation or microwaves. Each of these prior art techniques of water measurement in various materials has the disadvantage that it has either a limited range of water content which can be accommodated or a limited range of materials which it can accommodate, or both. Furthermore, the sensitivity to other characteristics, the spatial resolution and the ease and speed with which the materials can be scanned are all problems with the prior art devices.
Many dielectric materials are nearly transparent to microwaves, but with microwaves it is difficult to make multi-wavelength measurement. Multi-wavelength measurements can be effected in the infrared spectrum, however, the dielectric sheets may be quite opaque and thus it is often necessary to make reflection rather than transmission measurements. The measurement of reflection is inherently a more difficult problem than the measurement of transmission. Also, attenuation and diffusion of infrared radiation due to scattering is a serious problem.
The use of microwaves to measure moisture content is disadvantageous because the microwaves are too long to permit spatial resolution on a millimeter scale. This millimeter resolution is the extent required in many industrial applications. Although microwave devices can handle a broad range of moisture content, they are quite insensitive at low moisture levels. Infrared devices provide good precision at low moisture levels but their use is restricted to such levels, usually less than 10% moisture content.
One approach used in the prior art to develop a moisture type gauge is represented by the U.S. Pat. No. 3,851,175 to Dahlin wherein a moisture gauging apparatus is provided which includes a tungsten light source having a filament voltage source capable of remote adjustment. The light source is capable of supplying radiation in both the 1.8 and 1.9 micron wavelength bands. The radiation from the source is collimated by an optical system and provides a beam of radiation containing both a first spectral band of radiation lying outside but near an absorption band of water and a second spectral band of radiation that lies within the absorption band of water. This radiation beam containing both spectral bands is directed onto paper where the moisture content is to be determined. A receiver is provided for detecting the radiation, either transmitted through the paper or backscattered from the paper and the information received is converted into an electrical signal indicative of the ratio of the energy received in the bands. A signal is also supplied for indicating the opacity of the paper which is combined with the ratio indication signal to provide a compensated moisture reading.
This type of device employed by Dahlin, which employs two wavelengths isolated by filters to correct moisture measurement for the variation in other sheet properties is confined to low moisture levels and light stock paper which is of course limiting in not only moisture level but also in the kind of materials which can be examined. It is also to be noted that this multi-wavelength example provides for the necessity for making reflection rather than transmission measurements.
There has been a recent attempt to provide for thin paper sheet moisture measurements using a submillimeter laser as described in "Paper Sheet Moisture Measurements in the Far Infrared" Conference Digest, Eighth International Conference on Infrared and Millimeter Waves by Boulay et al., December, 1983. These theoretical discussions were confined to the use of a single wavelength metallic guided-wave CH.sub.3 OH laser with hole coupled end mirrors which were pumped with a CO.sub.2 laser whereby the output of the laser was directed at normal incidence onto a paper sheet. This theoretical discussion provided only a measurement for normal incidence on a thin paper sheet with a quasi continuous wave laser (CW) output and is unable to take into account the changing conditions with respect to sheet and band dielectric materials other than thin paper and is thus limited with respect to solving the other problems discussed above. Another disadvantage of this approach is that the hole coupled end mirror arrangement used on the output of the CH.sub.3 OH laser prevents the laser output from being confined to a well-collimated beam which can be directed onto a small area of the material. Furthermore, the diffused nature of the hole coupled mirror output makes it difficult to use for scanning across a width of material.